Conventionally, various substrate protecting means have been generally used in production of printed wiring boards. For example, a resist is used in an etching process, and a solder resist is used during a soldering process. Also in production of a film printed wiring board (hereinafter abbreviated as “flexible printed circuit board”) which is mounted in, for example, a small-sized device, in a soldering process for component mounting, a solder resist is used to protect wiring which is independent of the soldering process. Specifically, a cover lay film or a cover coat has been used as the substrate protecting means such as a resist, a solder resist, or the like. The cover lay film is formed by punching out a polyimide film in a given shape and is then laminated on a flexible printed circuit board. The cover coat is formed by printing, on a flexible printed circuit board, ink which is made of a heat resistant material. The cover lay film or the cover coat, which serves also as a protective film for wiring after soldering, is required to have not only UL-approved flame retardancy but also bleedout resistance, plasticity, adhesiveness, bending resistance and flexibility such that the cover lay film or the cover coat is not cracked while being bent during incorporation, into any device, of the flexible printed circuit board on which the cover lay film or the cover coat is laminated, heat resistance during soldering, moisture resistance, and insulation properties.
A cover lay film is formed by punching out a polyimide film and is then laminated on a flexible printed circuit board. The cover lay film satisfies the above required characteristics and is currently most widely used. Meanwhile, such a cover lay film has the following problems: (i) costs for producing the flexible printed circuit board increase due to (a) an expensive mold necessary for punching and (b) manual positioning and combining of a punched-out film; and (ii) formation of a micropattern is difficult.
As a method for solving the above problems, a method has been proposed in which a liquid photosensitive resin composition or a film photosensitive resin composition is applied to a substrate. According to the method, a micropattern can be easily formed in a case where exposure, development, and heating are carried out by a photographic technique after a photosensitive resin composition film is formed on the substrate. Therefore, various photosensitive resin compositions have been developed.
For example, Patent Literature 1 has proposed an alkali-developable photosensitive thermosetting resin composition which contains no halogen, has high-level flame retardancy, and is capable of forming a film that has a low warping property after curing and is excellent in plasticity, resolution, resistance to heat of soldering, chemical resistance and the like, the alkali-developable photosensitive thermosetting resin composition containing (i) a resin component which has a (meth)acryloyl group and a carboxyl group within a molecule and is soluble in a dilute alkali solution, (ii) an epoxy compound having two or more epoxy groups per molecule, (iii) a photopolymerization initiator, (iv) an organic phosphorus compound, (v) a diluent, and (vi) a polyimide resin having a predetermined structure. Meanwhile, Patent Literature 2 has proposed a photosensitive thermosetting resin composition which sufficiently satisfies flexibility and is also sufficiently excellent in flame retardancy and hot pressing resistance, the photosensitive thermosetting resin composition containing (i) a polymer having a carboxyl group, (ii) a photopolymerizable compound having an ethylenically unsaturated bond and containing no halogen atom, (iii) a photopolymerization initiator, (iv) a phenoxyphosphazene compound, (v) a phosphoric ester compound, and (vi) a halogen-based flame retardant.
A flexible printed circuit board is a wiring board having a thickness of several tens of μm to several hundreds of μm. Therefore, in case of component mounting, a stiffener having a thickness of 0.5 mm to 2.0 mm may be partially attached to a base material surface (see Patent Literature 3) or a non-photosensitive cover lay film surface (see Patent Literature 4) of a flexible printed circuit board so that a supporting property of a mounting part is secured. (a) of FIG. 3 is a schematic view of a stiffener-integrated flexible printed circuit board described in Patent Literature 3. (b) of FIG. 3 is a schematic view of a stiffener-integrated flexible printed circuit board described in Patent Literature 4. According to the stiffener-integrated flexible printed circuit board of Patent Literature 3, a stiffener (6), a bonding material (5), a flexible printed circuit board (1), and a non-photosensitive cover lay film (4′) are laminated in this order (see (a) of FIG. 3). According to the stiffener-integrated flexible printed circuit board of Patent Literature 4, a flexible printed circuit board (1), a non-photosensitive cover lay film (4′), a bonding material (5′), and a stiffener (6) are laminated in this order (see (b) of FIG. 3).